Along with the recent increase in the amount of traffic of vehicles, there have been some cases where tires of traveling vehicles puncture and cause serious accidents. Accordingly, it becomes important to properly keep the air pressure of tires of vehicles to prevent serious accidents.
In the United States, there are legal regulations for safety of tires to ensure safety during the travel of vehicles, and new motor vehicles must be equipped with a tire pressure monitoring system (TPMS). Some European countries and South Korea as well as the United States have already established legal regulations for safety of tires, and China and India are expected to start such legal regulations within a few years.
The tire pressure monitoring system is arranged inside a tire and requires a power source for driving. Primary batteries such as coin-type lithium primary batteries are generally used as the power source. However, when primary batteries are used as the power source of the tire pressure monitoring system, the batteries are exhausted if an operation time is long, and they need to be replaced.
To cope with this, as a configuration that does not require the replacement of batteries, a configuration is known in which power is generated by piezoelectric elements, solar batteries, electromagnetic induction or the like, and the power obtained thereby is supplied to the system, as indicated by Patent Documents 1 to 3, for example.
Meanwhile, as indicated by Patent Document 4, for example, a configuration is known in which a lithium-ion secondary battery is used as part of the power source of the system. In the configuration disclosed in Patent Document 4, power generated by a power generation device is accumulated in the secondary battery.